In recent years, most of the modular data center market has revolved around the form factor of a shipping container. The main advantage of this form factor is to allow for easy transportation of prefabricated data center modules, complete with their power and cooling subsystems, as well as their computing machinery. This approach allows for rapid and cost effective deployment of self-contained computing infrastructure. It is particularly interesting and effective if this infrastructure needs to be redeployed multiple times, for instance in military operations. It's downside, however, is that the dimensions of a standard shipping container can greatly constrain the layout of the computing machinery inside the modules, and can make the access to individual computing cabinets and subsystems more difficult, which in turn impacts the general maintainability of the modular data center.
Inside these data center modules, when the computing cabinets are arranged into rows, the width of a standard shipping container, typically 8-foot, allows for a single row of typically 42-inch deep cabinets, juxtaposed side-by-side. This typical cabinet size barely leaves 2-feet on each side of the row to make aisles for accessing the equipment. One way to circumvent this limitation is to place the computing cabinets on sliding rails to move them forward or backward for maintenance, which is hardly practical, albeit possible.
Another way to circumvent this limitation is by juxtaposing two shipping containers side-by-side and removing the party wall that separates the two containers. In this way, two facing cabinet rows can be formed with enough space for three 3-foot-wide aisles that is typically two lateral cold-aisles and one central hot-aisle. But the next problem is to be able to fit all of the other air handling and air conditioning subsystems within a relatively small space. And because of this small space, many smaller power and cooling subsystems must be integrated which in turn can make the data center costlier, less efficient, and less reliable.
Another popular idea is that of stacking multiple data centers in shipping containers, one on top of the other, to maximize the amount of computing capacity per unit of real-estate. This approach, however, compounds the accessibility problem, and does nothing to reduce the number of components and reliability issues that can stem from it. Redundancy is good, but having too many smaller components instead of a smaller number of larger components can be detrimental to efficiency and reliability. Moreover, making a heap of small data centers does not provide the same flexibility as having one large data center.
Finally, in the context of high power density computing cabinets, for high performance computing, there is a certain misconception about thinking that water needs to be brought all the way to the computing cabinet, even sometimes all the way to the main boards and processors. Even though it is true that carrying heat in water is much more energy efficient than carrying the same amount of heat into air, using water cooling for high performance computing also has its drawbacks, mostly cost and risk of leaks.